The disclosure of Japanese Patent Application No. 2001-116175 is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a control apparatus for an automatic transmission, and more particularly to an art for smoothly performing a speed-change operation requiring the release of two engagement elements and engagement of two engagement elements (simultaneous changeover of the four elements) between speed-change stages.
2. Description of Related Art
An automatic transmission is designed to change over power-transmission paths extending through speed-change elements composed of planetary gears by engaging or releasing frictional engagement elements and to establish a plurality of speed-change stages by changing a gear ratio. For the purpose of performing engagement and release of the engagement elements during a speed-change operation through the simplest hydraulic-pressure control while inhibiting generation of a speed-change shock, operations of the engagement elements for upshift or downshift are generally performed as follows. That is, basically, for a plurality of engagement elements or a single engagement element which are or is engaged to establish a certain speed-change stage, another one of the engagement elements is engaged additionally, or one engagement element that is in engagement is released. A so-called changeover operation of the engagement elements is performed if it is inevitable for reasons of the construction of a gear train. That is, while the engagement elements that are in engagement are released, the other engagement elements are engaged.
In recent years, automatic transmissions have developed a tendency of adopting a multiple-stage construction in response to a demand for an improvement in driveability and energy conservation based on reducing the amount of fuel consumption. Such a tendency of adopting a multiple-stage construction in automatic transmissions is realized in general by adding a speed-increasing or speed-reducing stage based on an overdrive or underdrive gear to a speed-change mechanism composed of a multiple-stage planetary gear set. As an alternative, however, there is also an art disclosed in Japanese Patent Application No. 4-219553. According to this art, a multiple-stage construction is achieved by two systems, namely, high and low systems for inputting power to a planetary gear set of a Ravigneaux type.
In the aforementioned gear train having a multiple-stage construction, the possibilities of selecting speed-change stages suited for a running condition of the vehicle are widened. Therefore, the changeover operation of engagement elements is not limited to a simple changeover of two elements but may be a complicated changeover of four elements if necessary. A so-called skip speed-change operation for shifting to a specific one of a multitude of speed-change stages at a stretch is an example requiring such changeover of four elements. In any case, in the case where such multiple changeover of four engagement elements is carried out, it is a serious problem as to how to control the sequence in which the engagement elements are engaged or released and timings at which the engagement elements are engaged or released. If this control is performed in a certain manner, a speed-change operation performed in the speed-change mechanism does not proceed smoothly or continuously. As a result, problems such as the occurrence of a phasic shock during the speed-change operation, an extraordinarily great shock at the end of the speed-change operation, and an unnecessarily prolonged speed-change period are caused.
The invention thus provides a speed-change control apparatus for an automatic transmission capable of preventing an increase in an interval between speed-change operations while preventing the occurrence of a shock during a speed-change operation requiring engagement and release of four engagement elements, by ensuring the smooth proceeding of the speed-change operation.
In various embodiments of the control apparatus for an automatic transmission where an operation of four engagement elements is acquired at the time of a speed-change operation for a first speed-change stage to the second speed-change stage, wherein the first speed-change stage is established through engagement of a first engagement element and a second engagement element and wherein a second speed-change stage is established through engagement of a third engagement element and a fourth engagement element, the control apparatus includes a speed-change control device for controlling a state of the second engagement element prior to the start of release of the second engagement element in accordance with a predetermined target rotational acceleration.